A phased array antenna uses multiple radiating elements to transmit, receive, or transmit and receive radio frequency (RF) signals. Phased array antennas are used in various capacities, including communications on the move (COTM) antennas, satellite communication (SATCOM) airborne terminals, SATCOM mobile communications, and SATCOM earth terminals. The application of mobile terminals typically requires the use of automatic tracking antennas that are able to track the beam in azimuth, elevation, and polarization to follow the satellite position while the vehicle is in motion. Moreover, the antenna should be “low-profile,” small and lightweight, thereby fulfilling the stringent aerodynamic and mass constraints encountered in the typical mounting.
One well known type of phased array antenna is an electronically steerable phased array antenna. The electronically steerable phased array antenna has full electronic steering capability and is more compact and lower profile than a comparable mechanical phased array antenna. The main drawback of fully electronic steering is that the antenna usually requires the integration of a lot of expensive analog RF electronic components which may prohibitively raise the cost for commercial applications. A typical electronically steerable phased array antenna comprises an assembly of phase shifters, power splitters, power combiners, and quadrature hybrids. Additionally, a typical electronically steerable phased array requires at least a few of these components at every element in the phased array, which increases the cost and complexity of the architecture.
In a typical prior art embodiment and with reference to FIG. 1, a phased array antenna 100 comprises a radiating element 101 that communicates dual linear signals to a hybrid coupler 102 (either 90° or 180°) and then through low noise amplifiers 103, 104. Furthermore, the dual orthogonal signals are individually phase adjusted by phase shifters 105, 106 before passing through a power combiner 107. In addition, the typical components in a phased array antenna are distributed components that are frequency sensitive and designed for specific frequency bands.
Phase shifters are used in a phased array antenna in order to steer the beam of the signals by controlling the respective phases of the RF signals communicated through the phase shifters. A typical digital phase shifter uses switched delay lines, is physically large, and operates over a narrow band of frequencies due to its distributed nature. Another typical digital phase shifter implements a switched high-pass low-pass filter architecture which has better operating bandwidth compared to a switched delay line but is still physically large.
Also, the phase shifter is often made on gallium arsenide (GaAs). Though other materials may be used, GaAs is a higher quality material designed and controlled to provide good performance of electronic devices. However, in addition to being a higher quality material than the other possible materials, GaAs is also more expensive and more difficult to manufacture. The typical phased array components take up a lot of area on the GaAs, and result in higher costs. Furthermore, a standard phase shifter has high RF loss, which is typically about n+1 dB of loss, where n is the number of phase bits in the phase shifter. Another prior art embodiment uses RF MEMS switches and has lower loss but still consumes similar space and is generally incompatible with monolithic solutions.
Quadrature hybrids or other differential phase generating hybrids are used in a variety of RF applications. In an exemplary embodiment, quadrature hybrids are used for generating circular polarization signals, power combining, or power splitting. In an exemplary embodiment, the outputs of a quadrature hybrid have equal amplitude and a nominally 90° phase difference. In another typical embodiment, the quadrature hybrid is implemented as a distributed structure, such as a Lange coupler, a branchline coupler, and/or the like. A 180° hybrid, such as a magic tee or a ring hybrid, results in a nominally 180° phase shift. In general, quadrature hybrids and 180° hybrids are limited in frequency band and require significant physical space. Additionally, since the structures are distributed in nature, their physical size increases with decreasing frequency. Moreover, the quadrature hybrids and 180° hybrids are typically made of GaAs and have associated RF power loss on the order of 3-4 dB per hybrid when used as a power splitter, and an associated power loss of about 1 dB when used as a power combiner.
In-phase power combiners and in-phase power splitters are also used in a variety of RF applications. In an exemplary embodiment, the outputs of an in-phase hybrid have equal amplitude and a substantially zero differential phase difference. In another exemplary embodiment, the inputs of an in-phase hybrid configured as a power combiner encounter substantially zero differential phase and amplitude shift. In a prior art embodiment, the in-phase hybrid is implemented as a distributed structure such as a Wilkinson coupler. In general, an in-phase hybrid is limited in frequency band and requires significant physical space. Additionally, since the structure is distributed in nature, the physical size increases with decreasing frequency. The in-phase hybrid is typically made of GaAs. Moreover, the in-phase hybrid generally has associated RF power loss on the order of 3-4 dB per hybrid when used as a power splitter and an associated RF power loss of about 1 dB when used as a power combiner.
In addition to the different components in a phased array antenna, an antenna signal can have different polarizations, namely linear, elliptical, or circular. Linear polarization consists of vertical polarization and horizontal polarization, whereas circular polarization consists of left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP). Elliptical polarization is similar to circular polarization but occurs with different values for the vertical and horizontal component magnitudes or if the phase difference between the vertical and horizontal components is a value other than 90°.
Conventional antennas utilize a fixed polarization that is hardware dependant. The basis polarization is generally set during installation of the satellite terminal, at which point the manual configuration of the polarizer hardware is fixed. For example, a polarizer is generally set for LHCP or RHCP and fastened into position. To change polarization would require unfastening the polarizer, rotating it 90° to the opposite circular polarization, and then refastening the polarizer. Clearly this could not be done with much frequency and only a limited number (on the order of 10 or maybe 20) of transceivers could be switched per technician in a given day.
Unlike a typical prior art single polarization antenna, some devices are configured to change polarizations without disassembling the antenna terminal. As an example and with reference to FIG. 2, a prior embodiment is the use of “baseball” switches to provide electronically commandable switching between polarizations. As can be understood by the block diagram, the rotation of the “baseball” switches causes a change in polarization by connecting one signal path to a waveguide while terminating the other signal path. However, each “baseball” switch is physically large and requires a separate rotational actuator with independent control circuitry, which increases the cost of the device such that this configuration is typically not used in consumer broadband terminals.
Furthermore, another approach is to have a system with duplicate transmit and receive hardware for each polarization. The polarization selection is achieved by maintaining the path of the desired signal and deselecting the undesired signal. However, doubling the hardware greatly increases the cost of the terminal. In yet another embodiment, a system may implement solid state diode or FET-based switches. The use of these electronic components may lead to high loss and limited power handling in microwave and mm-wave applications. These alternatives are size, power, and cost prohibitive for most applications, including phased arrays and low cost commercial applications.
Additionally, typical phased array antennas only form a single beam at a time and are often not capable of switching polarization. In order to form additional beams and/or have polarization switching ability from the same radiating aperture, additional phase shifting and power splitting or combining components are required at every radiating element. These additional components are typically distributed in nature, require significant physical space, are lossy, and only operate over relatively narrow frequency bands. For these reasons, polarization agile, multiple beam phased array antennas that can operate over multiple frequency bands are difficult to realize in practice.
Thus, a need exists for a phased array antenna architecture that is not frequency limited or polarization specific, and that is reconfigurable for different polarizations and able to transmit and/or receive over multiple frequencies and form multiple beams. Furthermore, the antenna architecture should be able to be manufactured on a variety of materials and with no associated RF loss. Also, a need exists for a phased array antenna that uses less space than a similar capability prior art architecture, is suitable for a monolithic implementation, and has components with a physical size that is independent of operating frequency.